The past progressive tense (also sometimes called "past continuous") is formed by using was/were + main verb + ing, as in, "We were eating dinner when the phone rang."
We use the past progressive tense to talk about actions that were already in progress when another action took place in the past. (At the moment the phone rang, we were already in the process of eating dinner.)
It's also common to use this tense to talk about two on-going actions that were happening at the same time in the past, as in, "While Justin Timberlake was finishing his new album, his fans were waiting for it to come out."
Finally, we can also use this tense to talk about actions in the past that took place over time, as in, "I couldn't sleep last night because dogs were barking the whole night."
Past Progressive Tense Grammar Quiz
Past Progressive Tense Lessons: